Apparatus and methods for cooling turbine bucket platforms

ABSTRACT

A turbine bucket includes a platform interfacing between an airfoil and a shank. The platform is provided with a plurality of cavities covered by impingement cooling plates along the platform underside. Purge air supplied in the gaps between adjacent buckets flows through holes in the impingement plates to impingement cool opposite wall portions of the platform. The cooling air in the cavities is transmitted through film cooling holes in the platform to form a thin film of insulating air along the surface of the platform exposed to the hot gas in the hot gas path.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to cooling turbine bucket platforms at the interface between the turbine airfoils and the shanks of the buckets, and particularly relates to apparatus and methods for flowing a cooling medium for impingement and convective cooling of the bucket platform.

Over the years, gas turbines have trended towards increased inlet firing temperatures to improve output and engine efficiencies. As gas path temperatures have increased, bucket platforms have increasingly exhibited distress including oxidation, creep and low cycle fatigue cracking. In certain turbines, temperature inlet profiles have become such that the platforms are seeing close to the peak inlet temperatures for the blade row. This exacerbates the potential distress on bucket platforms as these blades run even hotter.

Many prior bucket designs did not require active cooling of the platform due to low firing temperatures. Also, film cooling carryover from upstream nozzle side walls tended to lower the temperatures near the platforms from the resulting “pitch-line bias” of the inlet temperature profile. Newer bucket designs have utilized film cooling by drilling holes through the platform and using compressor discharge air to flow a layer of film air on the platform surface exposed to the hot gas path, protecting it from the high flow path temperatures. This, however, is limited to areas where there is sufficient pressure to inject the air to film cool surface portions of the platform exposed to the hot gas flow path. Many current bucket designs only have sufficient pressure to film cool the aft section of the platform where the gas flow path air has been accelerated to drop the local static pressure. Accordingly, there is the need to reduce the platform temperature to a level required to meet part life or durability requirements including oxidation, creep and low cycle fatigue cracking.

In accordance with the preferred aspect of the present invention, there is provided a turbine bucket having an airfoil, a shank, and a platform at an interface between the bucket shank and the bucket airfoil, the platform including at least one cavity closed by an impingement plate along an underside of the platform and spaced from an opposite wall portion of the platform, the opposite wall portion having a surface exposed to and in part defining a hot gas flow path through the turbine, the impingement plate including a plurality of impingement cooling holes for directing a cooling medium through the holes into the cavity and toward the opposite wall portion to impingement cool the platform, and a plurality of holes through the wall portion in communication with the cooling medium within the cavity for film cooling the platform surface exposed along the hot gas flow path.

In accordance with a further preferred aspect of the present invention, there is provided a method of cooling a platform of a turbine bucket comprising the steps of: (a) providing a cavity within the platform; (b) closing the cavity by securing an impingement plate along an underside of the platform spaced from an opposite wall portion of the platform, the opposite wall portion having a surface exposed to and in part defining a hot gas flow path through the turbine; (c) flowing a cooling medium through cooling holes formed in the impingement plate into the cavity and directing the cooling medium towards the opposite wall portion to impingement cool the platform; and (d) passing the cooling medium within the cavity through film cooling holes in the wall portion to film cool the surface of the platform along the hot gas flow path.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a perspective view of a bucket according to an example of the present invention;

FIG. 2 is a cross sectional view thereof, taken generally about on line 2-2 of FIG. 1 illustrating impingement plates and film cooling holes in exploded juxtaposition relative to cavities in the platform; and

FIG. 3 is a schematic representation of the convective and impingement cooling flows.

DETAILED DESCRIPTION OF AN EXAMPLE OF THE INVENTION

Referring to the drawings, particularly to FIG. 1, a turbine bucket, generally designated 10 includes an airfoil 12 and a shank 14 with a platform 16 interfaced between the airfoil 12 and shank 14. The airfoil 12, of course, extends radially outwardly from the platform 16 and includes leading and trailing edges 18 and 20, respectively. Below the shank 14 is a dovetail 22 forming part of the base of the bucket. It will be appreciated that the buckets 10 are arranged in a circumferentially spaced array thereof in generally correspondingly shaped dovetail grooves in the rim of a turbine rotor wheel, not shown. So-called angel wing seals 24 are provided on the forward and aft faces of the shank 14 of the bucket 10 for sealing purposes as is conventional. Typically, a cooling medium, e.g., compressor discharge air, is provided in the base of the bucket and circulated along the bucket shank and through the airfoil to cool the airfoil. The cooling medium typically discharges into the hot gas flow path through the tip of the airfoil and/or along the trailing edge. It will be appreciated that the wheel spaces are also purged by compressor discharge air which extends into recesses 15 between adjacent buckets. This purge air provides a supply of cooling air for the present platform cooling system. The pressure ratio between the static pressure in the shank recesses and the static pressure in the gas path is sufficient to provide a cooling air flow for convectively cooling the platform and also to provide a cooling air film, i.e., an insulating cooling film along the surface of the platform exposed to the hot gas flow path.

Particularly, the underside of the platform 16 is provided with one or more cavities in communication with the shank recesses 15 and which cavities have side walls in part defining the shape of the airfoil along the underside of the cavity. For example, and as illustrated in FIG. 2, a first cavity 30 is formed along the underside of platform 16 and in part defines a margin of the pressure side of the airfoil 12. That is, the cavity 30 includes an interior wall 32 which when cavity 30 is formed, forms a continuation of the pressure side wall of the airfoil 20 along the underside of the platform. The cavity 30 is also defined by a leading edge wall portion 34 and a slash face wall portion 36.

Additional cavities are provided along the underside of the platform 16 which likewise in part define margins of the airfoil. For example, cavity 38 is formed in the underside of platform 16 and has an inner wall 40 which defines a continuation of the suction side wall of the airfoil 12 adjacent the airfoil trailing edge 20. The cavity 38 is also defined by a trailing wall 42 and a slash face wall 44. Similarly, a third cavity 46 is formed in the undersurface of the platform 16 adjacent a leading edge of the platform and forms, in part, a continuation of the suction side of the airfoil into the platform underside. The cavity 46 thus includes a wall portion 48 defining a margin of the suction side of the airfoil surface as well as wall portions 50 and 52 forming portions of the forward edge and slash face, respectively, of the platform.

The cooling medium passages illustrated in FIG. 2 preferably have a serpentine configuration. That is the airfoil cooling passages extend through the platform and alternate in a continuous passageway generally radially outwardly and inwardly through and along the airfoil. The various passages through the airfoil, form the cooling circuit for the bucket airfoil and do not form part of the present invention. Suffice to say, the airfoil cooling circuit may be closed, e.g. where steam is supplied through the serpentine passages in the airfoil for cooling purposes and returned. The airfoil cooling circuit may also be open where the cooling medium, e.g., air flows through the passages for discharge at the airfoil tip and/or along the airfoil trailing edge.

As can be seen in FIG. 2, each of the cavities 30, 38 and 46 include an impingement cover plate. For example, cavity 30 includes an impingement cover plate 54 which underlies cavity 30. Cavity 38 has an impingement cover plate 56 underlying cavity 38 and cavity 46 has an impingement plate 58 underlying cavity 46. The impingement plates are spaced from opposite wall portions 55, 57 and 59 of the respective cavities. As illustrated, each of the impingement plates 54, 56 and 58 has impingement holes 60, 62 and 64, respectively. It will be appreciated that the impingement holes transmit the cooling medium, e.g. purge air within the recesses 15 between the buckets, into the associated cavities for impingement cooling flow against the back side of the platform, i.e., the opposite wall portions 55, 57 and 59 of the platform. These impingement flows form jets of cooling air which produce a high heat transfer coefficient on the backside of the platform to both impingement and convectively cool the surface of the platform exposed to the hot gas flow path. Once the air flow has impinged on the backside of the platform, as illustrated by the arrows 68 in FIG. 3, the cooling medium flows within the cavities to convectively cool the platform surface as illustrated by the arrow 70. Next, the flow is discharged through the platform via film cooling holes in each of the cavities, for example, holes 72, 74, and 76 in cavities 30, 38 and 46, respectively. These cooling holes form a film or layer of air along the surface of the platform exposed to the hot gas in the hot gas flow path. This cooling layer insulates the bucket platform from the hot flow path gas.

It will be appreciated that the impingement plates may be securely fastened to the platforms by brazing, welding or another type of mechanical securement. As illustrated, the film cooling air flow occurs along the platform surface exposed to the hot gas adjacent the pressure and suction sides of the airfoil.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A turbine bucket having an airfoil, a shank, and a platform at an interface between the bucket shank and the bucket airfoil, said platform including at least one cavity closed by an impingement plate along an underside of said platform and spaced from an opposite wall portion of said platform, said opposite wall portion having a surface exposed to and in part defining a hot gas flow path through the turbine, said impingement plate including a plurality of impingement cooling holes for directing a cooling medium through said holes into said cavity and toward said opposite wall portion to impingement cool said platform, and a plurality of holes through said wall portion in communication with the cooling medium within said cavity for film cooling the platform surface exposed along the hot gas flow path.
 2. A turbine bucket according to claim 1 wherein said cavity is located in said platform adjacent a pressure side of said bucket.
 3. A turbine bucket according to claim 1 wherein said cavity is located in said platform adjacent a suction side of said bucket.
 4. A turbine bucket according to claim 1 including a second cavity within the platform closed by a second impingement plate along an underside of said platform and spaced from a second opposite wall portion of said platform, said second opposite wall portion having a second surface exposed to and in part defining the hot gas flow path through the turbine, said second impingement plate including a plurality of impingement cooling holes for directing a cooling medium through said holes thereof into said second cavity and toward said second opposite wall portion to impingement cool said platform, and a plurality of holes through said second wall portion in communication with the cooling medium within said second cavity for film cooling the second platform surface exposed along the hot gas flow path.
 5. A bucket according to claim 4 wherein said first and second cavities are located in said platform adjacent a suction side of said bucket airfoil.
 6. A bucket according to claim 4 wherein said first and second cavities are located in said platform adjacent respective suction and pressure sides of said bucket airfoil.
 7. A turbine bucket according to claim 1 including second and third cavities within the platform closed by said second and third impingement plates along an underside of said platform and spaced from opposite second and third wall portions of said platform, said second and third opposite wall portions having respective surfaces exposed to and in part defining the hot gas flow path through the turbine, said second and third impingement plates including a plurality of impingement cooling holes for directing a cooling medium through said holes into said second and third cavities, respectively, and toward said second and third opposite wall portions to impingement cool said platform and a plurality of holes through said second and third wall portions in communication with the cooling medium within said second and third cavities for film cooling the respective platform surfaces exposed along the hot gas flow path.
 8. A turbine bucket according to claim 7 wherein said one cavity is located in said platform adjacent a pressure side of said bucket airfoil and said second and third cavities are located in said platform adjacent a suction side of said bucket airfoil.
 9. A method of cooling a platform of a turbine bucket comprising the steps of: (a) providing a cavity within the platform; (b) closing the cavity by securing an impingement plate along an underside of the platform spaced from an opposite wall portion of the platform, said opposite wall portion having a surface exposed to and in part defining a hot gas flow path through the turbine; (c) flowing a cooling medium through cooling holes formed in the impingement plate into said cavity and directing the cooling medium towards said opposite wall portion to impingement cool said platform; and (d) passing the cooling medium within said cavity through film cooling holes in said wall portion to film cool the surface of the platform along the hot gas flow path.
 10. A method according to claim 9 including providing said cavity within the platform adjacent a pressure side of the bucket airfoil.
 11. A method according to claim 9 including providing said cavity within the platform adjacent a suction side of the bucket airfoil.
 12. A method according to claim 9 including providing a second cavity within the platform, closing the second cavity by securing a second impingement plate along an underside of the platform spaced from a second opposite wall portion of the platform, said second opposite wall portion in part defining the hot gas flow path through the turbine, flowing the cooling medium through cooling holes formed in the second impingement plate into said second cavity and directing the cooling medium toward said second opposite wall portion to impingement cool said platform, and passing the cooling medium within said second cavity through film cooling holes in said opposite wall portion of said second cavity to film cool the surface of the platform exposed along the hot gas flow path.
 13. A method according to claim 12 including providing the first and second cavities in said platform adjacent respective pressure and suction sides of said bucket airfoil.
 14. A method according to claim 12 including providing said first and second cavities in said platform adjacent respective pressure and suction sides of said bucket airfoil.
 15. A method according to claim 9 including providing second and third cavities within the platform, closing the second and third cavities by securing respective second and third impingement plates along an underside of the platform spaced from second and third opposite wall portions of the platform, said second and third opposite wall portions have respective second and third surfaces exposed to and in part defining the hot gas flow path through the turbine, flowing cooling medium through the cooling holes formed in the second and third impingement plates into said second and third cavities and directing the cooling medium toward said second and third opposite wall portions, respectively, to convectively cool said platform, and passing the cooling medium within said second and third cavities through film cooling holes in said second and third opposite wall portions, respectively, to film cool the surfaces of the platform exposed to the hot gas flow path. 